Command Explanations

--prefix=$(pkg-config --variable=prefix
ORBit-2.0): Setting the prefix using this parameter
instead of with $GNOME_PREFIX will
ensure that the prefix is consistent with the installation
environment and the package will be installed in the correct
location.

--sysconfdir=/etc/gnome/2.18.3: This
parameter causes the configuration files to be installed in
/etc/gnome/2.18.3 instead of
$GNOME_PREFIX/etc. Additionally (if
applicable), the parameter ensures that the GConf-2 database is correctly updated.

--mandir=$(pkg-config --variable=prefix
ORBit-2.0)/share/man: This parameter causes the
man.(X) files to be installed in
$GNOME_PREFIX/share/man/man(X)
instead of $GNOME_PREFIX/man/man(X).

--with-at-spi-registryd-directory=...:
This parameter is used to identify where the AT-SPI Registry daemon
is located, as the default is $GNOME_PREFIX/libexec.